The present invention relates in general to identifying signal quality of a received radio transmission, and more specifically to a stop circuit for terminating a scan tuning operation when a transmission is being received having a signal-to-noise ratio greater than a predetermined ratio. The present application is related to U.S. application Ser. No. (88-540) filed concurrently herewith.
The measurement of received signal quality has been employed in FM receivers for implementing a number of different functions. For example, automatic scan tuners which sweep through the FM broadcast band must terminate a scan when a broadcast signal is received having a particular signal quality. Signal quality is also measured in order to modify receiver characteristics in response to certain conditions, e.g., output blanking or reducing stereo separation in response to certain kinds of noise. Signal quality may also be measured in order to display a visual indication of received signal quality to the user of the receiver.
Several different methods have been employed to generate a stop signal for terminating the sweep operation in an automatically scanning tuner. Most methods rely on a quality measure based on the received signal strength, or a frequency window determined by the automatic frequency control (AFC) signal, or a combination of both. Detecting the presence of a received signal strength as being greater than a predetermined magnitude is, by itself, a poor indicator of signal quality. This is because the received signal strength may be high while signal quality is low due to the presence of noise, adjacent channel interference, or multipath interference. The use of a frequency window to ensure that scanning is stopped accurately at the frequency of a strong received signal does not eliminate the possibility of stopping at a received signal including noise and interference.
It is also known to examine the noise component of a signal relative to the information signal component in order to determine signal quality. For example, communications transceivers employ output inhibiting or squelch when a particular level of noise is present. However, the prior art systems measure noise at frequencies that also contain information-related components. Therefore, only noise that is substantially equal to or greater than the information component can be detected.
The measurement of the actual signal-to-noise ratio of a received signal requires complicated and expensive systems which can determine distinct power values for different frequency ranges and generate a ratio of the power values. Simplified systems are known for FM receiver applications which measure the difference in power values in order to identify a signal quality level. For example, Schiebelhuth et al, U.S. Pat. No. 3,889,192, describes a noise suppressing arrangement which determines the absolute levels of the received field strength and the receiver noise. The audio output is enabled only when the field strength is above a predetermined value and the noise is below another predetermined value. Thus, whenever the audio output is enabled, a received signal has at least a minimum signal-to-noise ratio. However, not all signals having that minimum signal-to-noise ratio will be accepted by the system. For example, a signal having a field strength slightly below the predetermined field strength but having substantially no noise would result in the audio output being inhibited. Similarly, a signal having a noise component slightly in excess of the predetermined noise value but having a very large field strength would also not be output by the receiver.